I stood at the end of the frozen runway, peering through the mist, trying to make out the terminal building in the distance.

It was exactly at this spot, and under the cover of darkness, that the CIA planes did their business.

"They always followed the same procedure," says Mariola Przewlocka, the manager at the remote Szymany airport in north-east Poland when the strange flights arrived during 2003.

"We were always told to keep away. The planes would stay at the end of the runway, often with their engines running. A couple of military vans from the nearby intelligence base would go up to them, stay a while and then drive off, out of the airport.

'Cash payments'

"I saw several of these flights but never saw inside the vans because they had tinted windows and they never stopped at the terminal building.

"Payment was always made in cash. The invoices were made out to American companies but they were probably fake," says Mrs Przewlocka.

European MPs visited Poland to investigate the claims

In September 2006, President Bush admitted what had been suspected for a long time - that the CIA had been running a special programme to transport and interrogate leading members of al-Qaeda, away from the public spotlight.

Human rights groups have expressed concerns that the prisoners may have been tortured.

The hunt has been on ever since to locate the secret prisons, or "black sites" as they are known.

Poland and Romania have been named by investigators as hosting such sites.

The claims are denied by both governments.

CIA landings

After a week of meetings in smoky Warsaw restaurants and coffee bars with Polish intelligence sources, airport workers and journalists, I obtained what I had been looking for, and something that nobody in authority wanted to reveal, the flight log of planes landing at Szymany airport.

They confirmed my eyewitness's account - that a well-known CIA Gulfstream plane, the N379P, had made several landings at the airport in 2003.

The plane has been strongly linked to the transportation of al-Qaeda terrorists.

Another plane, a Boeing 737, had flown direct from Kabul to this remote Polish airport.

"There is no particular reason for a Gulfstream to stop there. So there has to be a reason why the plane is stopping there and the fact that everyone is trying to conceal this reason makes it all the more interesting to try to find out what it is," says Anne Fitzgerald from Amnesty International.

I followed the route of the military vans from the airport to the nearby secret Polish intelligence base at the village of Stare Kiejkuty.

He says: "It's almost a foregone conclusion that Poland hosted a CIA Black Site."

But the authorities in Poland do not want to talk about it.

All requests for interviews with government ministers were rejected. The European parliamentarians met a similar wall of silence.

One civil servant from the prime minister's office claimed a secret, internal inquiry had concluded there had been no "black site" in Poland.

Others disagree.

"I think it's quite probable there was a kind of transfer site, a black site, in Poland. There is a Kafka-like mood in Warsaw. No one from the government has the will to answer our questions," says Jozef Pinior, a senior Polish politician, who has called for a commission to investigate the claims.

With Polish troops serving in Afghanistan and Iraq, and with the United States as the country's key ally, there is no desire to delve into the secret deals made in the secret war against international terrorism.

The US state department has said it always complies with its laws and treaty obligations and respects the sovereignty of other countries.

But the truth of Poland's role may soon emerge.

The new Democratic-controlled US Congress may begin its own investigation into the CIA secret prisons programme in the next few months.

The search for Poland's secret CIA prison is broadcast in Global Account for the first time at 23.06 GMT on Thursday 28 December on BBC World Service.

A longer version of the same programme, "Chasing Shadows", will be broadcast on BBC Radio 4 at 20.00 on Tuesday 2 January , repeated Sunday 7 January at 17.00.